Reinforced propeller blade



Oct. 28, 1952 c. A. LIEDHOLM 2,615,520

' REINFORCED PROPELLER' BLADE Filed Nov. 26, 1947 INVENTOR.

ATTORNEY.

Patented Oct. 28, 1952 UNITED! STATES VTENYTV'V REINFoRcsn'PROBELLVERJBLADE Carl A. Liedholm, Mountain Lake, N. J assignor toCurtiss-Wright Corporation, a corporation of Delaware I ApplicationNovember 26, 1947, Serial No. 788,211

part to high localized stresses induced by vibration, andin part to theinevitable microscopic or macroscopic checks and cracks which may appearanywhere in the mass of the steel of which the blade is formed. Suchfailures are further contributed to by imperfections in the welding ofblade components to one another, .which allow, scattered zones wherelocalized stresses may be high enough to cause localfailures and tothereby initiate cracks which may grow to serious proportions as theblade continues to operate at high stress levels. These faults andcracks may occur despite precision techniques in the fabrication ofblades and despite zealous and careful inspection of allsurfacesof. thepropeller blade. It will berealized that the problem is of rathercritical nature since blades operate .at high stress levels, and the{mass of material of which they are composed is held to the minimumtoreduce the weight of the blade. I

I.'.;In spectionof the exterior of a fabricated propellerblade, prior tofinal surface finishing,v is comparatively simple magnetic, macroscopicand visual inspection can be made with little -difficulty. Few bladefailures result from flaws having their inception at the bladeoutersurface. :The interior surfaces of hollow steel propeller blades,however, are much more difficult to' inspect since remote inspectiontechniques must be practiced with the aid of optical and otherinstruments which maybe inserted into the hollow of the blade from thebutt end. I have found that most failures of propeller blades occur frominterior inceptions,'which are contributed to by inspection difficultiesmentioned above and to difficulties attendant to fine surface finishingof the interior surfaces of propeller blades.

;raising tolerable operating stress level and in =reducing thelikelihood of failure in propeller blades.

,-.- -Objects of this invention are to increasethe -1 Claimd; (o1.fin-r159) resistanceof hollow steel propeller blades to failures due tovibratory stresses'and to other stresses which may be imposed thereon,andto effect such resistance tofailure by internally precompressingeither all or part of the interior surface of a hollow steel propellerblade and increasing its strength and fatigue resistance-by a suitablemetallurgical technique such as by nitriding.

Further objects of the invention will become apparent from the followingdescription and claim,- and from the drawings. The drawings and thedescription: associated therewith, however, are not to be construed aslimiting the scope of the invention since various other means foraccomplishing objectives of the invention will become apparent to thoseskilled in the art;- a

. In the drawings in which similar reference characters representsimilar parts, Fig. l is a a v Fig. f is a section similar to Fig. 3 atthe blad trailing edge; and v I Fig. 5 is an enlarged fragmentarysection of a blade leadingedgeportion showing an alterna- .tive mode ofpracticing the invention.

The propeller blade to which the invention. is applicable is representedin its entirety in Fig. 1 and comprises a tubular shank portion l0provided if desired with a flange l2 utilized in connection withretention of the propeller blade in a propeller hub. The blade also maybe provided if desired with an annular cuifring l4 which provides anabutment against which a. blade embracing cuff may be secured againstdisplacement under the influence of centrifugal force. .These elements[2 and 14 do not form a partbf the invention and are illustrated merelyas examples of a particular blade organization. The eifective portion ofthe blade aerodynamically, comprises a large paddle shaped member 16constituted by a camber plate l8 and a thrust plate 20, thetwo platespreferably being tapered in-thickness longitudinally-of the blade-andhav- I and adjacent the trailing edge 24 are ordinarily thickened andare integrally united with one another through the medium of welds.Further, and particularly where blades are large] and wide, the blademay be centrally reinforced by a rib 26 extending longitudinally andcentrally of the blade and secured as by welding to thickened portions28 of the camber and thrust plates. Frequently, the rib 26 is formed inpart on each blade plate and the two parts are united along the bladeneutral axis to comprise the entire rib. As inferred, a rib such as 26may or may not be necessary in the blade, depending upon its size anddesign requirements.

In that type of blade wherein separate camber and thrust plates arepreformed and are subsequently united at their leading and trailingedges, the weld at said edges is applied from the outside of the bladeand a fillet 30 is formed within the leading edge, a similar fillet 32being formed internally of the blade adjacentthe trailing edge.- Theinterior surface of the welded fillet is ordinarily more or lessnon-uniform in its surface finish and unless it be finished smoothly asby filing or grinding, high and low points and other faults may exist onthe fillet as shown at 34 and 36 in Fig. 3, which under blade operatingconditions form points of high stress concentration which may lead tofatigue failure. The proce dures for securing a fine finish in bladefillets are rather costly and time consuming but up to this time arefully justifiable in view of the improved fatigue resistance of theblade which is produced. The fillets however, are not the only points atwhich. failures may initiate, and in certain types of blades, due to thepattern of stress distribution occasioned by the blade design andv bythe vibratory stresses and steady state stresses imposed upon the bladein operation, failures can occur at other-parts of the blade surface. Ananalogous situation to failures may also exist in blades where welds aremade in' other blade zones than the leading and trailing edges.

I have found that by superficially increasing the strength of andprecompressing the interior surface of a propeller blade to the properdepth, the effect of localized faults in the blade surface is minimizedand that the fatigue resistance of a propeller blade can be raised tolevels heretofore not obtainable. A preferred method of securing theprecompression indicated at 38 is to nitride the interior surfaces ofthe propeller blade. This may be easily accomplishedby'following'theusual nitriding procedure; namely and in general, thesoaking of thebla-de for anappropriate length of time at al:u )'ut-9l5'F. while-nitrogen carryin gas such as ammonia is passed over the surfaceto lie-nitrided. This forms iron nitride in the steel, increasing itsstrength and hardness and-causing a. slight growth of the material. Thisgrowth causes precompression of the material since it is constrained bythe enveloping unn'itrided blade 'material'. In the case of thehollowblade this technique is extremely simple-- in. itsaccomplishmentsince the blade proper may be soaked at the propertemperature in a. non-oxidizing atmosphere and ammonia gasmaybepassedinto the blade hollow through anappropriategas' connection.associated with the. butt" of the propeller blade. A. nitridedl casehaving reat fatigue strength and hardness, develops over the innersurface of the propeller blade toa depth offrom .005 to 030 inchdepending upon the time of soaking',.the inner margins of thecaserblending' gradually into the parent metal.

In some types of propeller blades, nitr'iding of the entire interiorsurfaceof the blade maybe unnecessary, in which case those portions ofthe blade surface where nitriding is notzdesired may 4 be coated with anappropriate plating which pre vents nitriding. The nitrlding may belocalized at such zones, for instance, as the fillets at the inside ofthe leading edge and the trailing edge of the blades as noted in Fig. 5at 4|).

The best depth of the precompressed or nitrided layer varies with bladesof difierent design. It should be a depth to allow substantially thesame high operating stress levels to be maintained both in the outer andinner surfaces of the blade material, such stress levels being themaximum to allow freedom from blade failure. Expressed in another way,the depth of the nitrided layer should be such that failures of theblade due to stress overload, normally applied to test samples, shouldstart from the inner surface and from the outer surface withsubstantially equal frequency. When such a condition prevails, the safeoperating stress level of the blade is at a maximum and is materiallygreater than that attainable with untreated blades, or with overorunder-treated blades. Even so, moderately overor undertreated bladesshould be better than untreated blades.

A considerable amount of testing has been accomplis'hed on propellerblades, otherwise identical, some having nitrided inner surfaces andothers being unnitrided. Actual nitrided blade samples under enforcedtest conditions have failed in a stress range of the order of a minimumof 40,000 pounds per square inch whereas the un-nitrid'ed samples havefailed at stresses of the order of a minimum of 23,000 pounds per squareinch, the latter failures having started from the blade interiorsurfaces.-

While nltriding of the steel of the propeller blade is referred as ameans for internal surface precompressing and, incidentally, hardeningand strengthening, other means may be employed for increasing theprestress, hardness and strength of the material, such as casecarburiz'ing and cold working. It is appreciated that the prior art hastaught the treatment of propeller blades to ditfei'entiate strength andhardness in the wall of the bladebut in all instances of which applicanthas knowledge, emphasis the art has been placed upon increasing thestrength and hardness of the exterior surfaces of the blades either byelectro-deposition of hard metal, cold working, surface carburization ornitriding. The primary purpose of external strengthening and hardeningwas to increase propeller blade resistance to abrasion and externalerosioh and naturally to increase the strength Of the blade in general.However, suchprior art-- expedients' while constru tivefcrimprovingabrasion resistance, do not provide a solution for the underlying roblms or reinforcing blades against vibration induced stress or ofequalizing failure incepticns between the outside and inside bladesurfaces. Nor do the prior teachings provide means to overcomeimperfections inthe interior of the blades where surface finishing. andinspection is extremely difiicult.

The phenomenon of increasin'gfatigue strength of propeller blades bynitridihg or other modes of internal hardening is believed to consist inthe proposition that the treated surfaces are placed in a stateofcontrolled compressive stress which delays fault inception where thesurface finish may be imperfect- It is known that the technique ofnitriding causes a growth of the material of the order of a few per centwhereby the nitrided layers are inevitably compressedand the metallayers adjacent thereto are correspondingly placed in tension. Thecontrolled compressive stress at the interior blade surface, by actualtest materially increases the fatigue strength of the propeller bladesdespite minor defects which may occur on the interior surfaces of theblade or on the interior surfaces of the relatively unfinished filletsor Welds which occur at various parts of the blade. When techniques suchas cold work or other hardening processes are used on the interiorsurfaces of the propeller blade, it is considered that such techniquesshould properly include the creation of a state of precompression on theinner blade surfaces.

Though two embodiments illustrating the invention have been shown anddescribed, it is to be understood that the invention may be applied inother and various forms. Changes" may be made in the arrangements,without departing from the spirit of the invention. Reference should behad to the appended claim for definitions of the limits of theinvention.

What is claimed is:

A hollow steel propeller blade comprising an integral, substantiallyhomogeneous shell of airfoil profile having thickened leading andtrailing edge portions and relatively thinner shell portionstherebetween, said thinner shell portions extending across the bladechord from a short distance rearward of the leading edge to a shortdistance forward of the trailing edge, the interior surface layers ofsaid leading and trailing edge portions 6 deviating from the homogeneityof the rest of the blade and being nitrided to a depth of from .005" to.030, to precompress said surface layers, and to increase the strengthand hardness thereof better to resist the action of operating stressesthereat, said nitrided surface layers terminating where the thickenedleading and trailing edge blade portions blend in thickness to said Ithin shell portions.

' CARL A. LIEDHOLM.

REFERENCES CITED The following references are of record in the MetalsHandbook, 1948 Ed., pub. by A. S. M., 7301 Euclid Ave., Cleveland, Ohio,p. 700.

